Offshore installations, such as offshore windmills, various process modules for subsea oil and gas exploration etc., are in many cases deployed by being transported on seagoing transport vessels out to the placement location, and thereafter lifted off the transport vessel by an on-board crane or crane located on another vessel and lowered into the sea, either to be located on the surface as a floating installation or lowered into the water to be installed on the sea floor.
The deployment, more precise, the lifting operation is sensitive to the weather conditions since, during suspended loads offshore, sea induced movements of the vessel will quickly become problematic since the movements of the vessel and thereby the crane results in variations in the available lifting height between the sea surface and the crane's point of suspension, and because the cyclic wave movements results in periodic accelerated movements of the suspended load.
This is particularly problematic for lifting of heavy cargoes and/or huge constructions, since the accelerated movements of the suspended load induces huge tension forces/strains on the crane and suspension points in the load and danger of re-contact between the load and the transport vessel which may damage the suspended load. After the load has obtained contact with the sea-water, the waves will become problematic since the sea-water viscosity and own weight retains the load/construction with enormous power when the movement of the vessel tries to increase the distance between sea surface and the suspension point of the crane.
Thus, to avoid costly waiting periods where the transport vessel is laying inactive waiting for improved wave conditions allowing such deploying operations, it is a desire for amending these wave induced problems allowing performing the deployment in less favourable weather conditions. The reduction in costs, due to saved waiting times, increases with increased “weather window”. It is thus a need for compensating the variation in available lifting height between the point of suspension of the crane and the surface of the sea due to wave induced movements of the lifting vessel to avoid detrimental tension forces on the crane, the suspended load, or re-contact between the vessel and load.